1. Field of the Invention
The present invention relates to methods and systems for operating computer memory devices.
2. Description of the Related Art
Non-volatile NAND-based flash memory systems are widely used in modem digital computing devices, such as mobile phones, digital cameras, digital camcorders, computers, and many other digital computing devices. Quality control in flash memory production requires detection and mitigation of flash defects such that the number of defective parts (memory chips) per million (DPPM) is maintained below a threshold value of DPPM set by quality standards. A defective flash memory device is one where the data stored in the memory device becomes unavailable, i.e., cannot be read from the memory device.
One factor that contributes to data loss in flash memory is data corruption caused by physical defects referred to as “flash defects.” The term “defect” is used herein to refer to such “flash defects.” A defect is of concern when the physical damage associated with the defect causes changes to the stored data that cannot be corrected by implemented protection mechanisms such as error correction code (ECC) processes. A given defect may cause corruption of data in either one word line or multiple word lines simultaneously. Also, defects are “grown” in the sense that a given defect develops when electrical field stress is applied to memory cells during read, program, and erase operations, until a breakdown point is reached where the defect has grown to the point that data cannot be retrieved from the memory cells affected by the defect. Upon reaching the breakdown point, the word line that contains the memory cell affected by the defect becomes unusable, and possibly other neighboring word lines also become unusable depending on the physical damage associated with the defect. It is within this context that the present invention arises.